For laser processing of workpieces, it is preferable to know the focal position of the laser beam relative to the workpiece surface. In a calibration step, a search is therefore normally effected for a focusing setting at which the focal point of the laser beam lies on the workpiece surface. This focal point corresponds to the reference focal position. During processing, focal position offsets are adjusted relative to this reference focal position as a function of the task to be performed (e.g., producing a pilot hole in the workpiece, cutting the workpiece, or marking the workpiece).
In some cases, changes in the distance between the processing optics and the focal point of the beam may occur in laser processing machines (e.g., high laser powers) due to, for example, contamination of the focusing optics as a result of heating of the optical components in the beamline. The focal position on the workpiece and the precision of the laser processing machine therefore may also change. During operation, it is therefore preferable to repeatedly check the focal position in order to be able to detect a variation from the optimum focal position at an early stage and correct it.
Examples of methods for deter mining the reference focal position of a laser beam can be found in the references DE 102 55 628 A1, JP 02160191, JP 10258382, JP10314966 and JP10076384, in which a plurality of cutting lines is produced on the workpiece surface with the laser beam with a varying focal position. The width of the individual cutting lines is subsequently determined and the kerf having the narrowest width is determined. The focal position associated with the narrowest cutting width is adopted as the focus setting of the laser processing machine. In the methods described in the first four of the above-mentioned specifications, the width of the cutting line is determined using a camera positioned preferably at the laser processing head; in the fifth reference, the line width is determined capacitively, by moving the laser processing head transversely to the cutting lines and in so doing measuring the change in capacity using a capacity sensor located at the laser processing nozzle of the laser processing head.
Apart from the precise adjustment of the focal position, the position of the focused laser beam as it passes through the opening of the nozzle body of the laser processing nozzle during laser processing should also be precisely set, depending on the application, to a few one-hundredths of a millimeter, since this facilitates achieving a directionally independent processing result. The opening in the nozzle body through which the laser beam passes is generally circular, but other forms such as, for example, slots, are also possible.
The base setting of the correlation of the focused laser beam with the center of the laser processing nozzle and with a predetermined exit position of the laser beam from the laser processing nozzle can be altered when using a new laser processing head or after exchanging or cleaning components of the laser processing head. The correlation can be verified during operation of the laser processing machine on the basis of cyclic measurements, or if the processing result deteriorates. Such verification also can be performed if there is a change of focus setting.
Alignment of the laser beam in the opening of the laser processing nozzle can be carried out manually. In some cases, this include securing an adhesive tape over the nozzle opening and burning a small hole into the adhesive tape at low laser power. The variation of the beam position of the laser beam from the center of the nozzle is determined by the naked eye and a magnifying glass. The beam position is corrected by appropriate positioning units with adjusting screws, through which the position of the laser beam in an X-Y plane of the opening in the nozzle body that is at right angles to the nozzle axis can be altered in the X-direction and the Y-direction. This process can be imprecise, time-consuming and unsuitable for an automated process sequence because of the manual interventions that may be required.
JP 06328281 discloses using a laser beam to cut a circular aperture in a workpiece to center the laser beam in the laser processing nozzle, in which the midpoint of the aperture corresponds to the midpoint of the laser beam intensity in a plane that is perpendicular to the propagation direction of the beam. The middle of the circular opening in the laser processing nozzle is determined by touching opposite edges of the aperture with the nozzle body, the contact between nozzle body and the edge being detected by the short-circuit of a voltage applied between the nozzle body and the workpiece. If the midpoints of the laser beam and the nozzle opening do not coincide, the beam is aligned to the midpoint of the opening in the laser processing nozzle. This method may possibly lead to inaccurate results if the alignment of the laser beam was very off-center at the start, as in this case the circular cut-out has very oblique edges, producing inaccurate results when these are touched by the laser processing nozzle.
WO 2006/027085 discloses determining the position of a laser beam axis relative to a beam axis of a process gas jet. When the process gas jet touches an edge of an axial position detection element, the contact is detected through a deflection of the axial position of the detection element. In particular, contact of the laser beam with the edge of the axial position detection element is detected by a process light measurement and, by comparison of the contact points determined in each case, the position of the axis of the laser beam relative to the position of the axis of the process gas jet can be determined.